Vitamin B12 injections deliver high doses of this essential vitamin directly into your bloodstream for better absorption than oral supplements to boost energy, support red blood cell production and maintain nerve function. How Quickly Does B12 Shot Work? Does It Help to Lose Weight? A vitamin B12 injection is a shot that delivers B12 directly into your muscle, helping your body absorb it quickly (especially if your body struggles to absorb it from food or tablets) B12 Injections vs Tablets: Key Differences Explained